The red mark beneath the gills of this hybrid trout is fainter than in the cutthroat, and its spots are more broadly distributed.

Clint Muhlfeld
/ USGS

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Originally published on June 10, 2014 8:53 am

Many parts of the U.S. have been getting warmer over the past several decades, and also experiencing persistent drought. Wildlife often can't adjust. Among the species that are struggling is one of the American West's most highly prized fish — the cutthroat trout.

In springtime, you can find young cutthroats in the tiny streams of Montana's Shields Basin. Bend over and look closely and you might see a 2-inch fish wriggling out from under a submerged rock — the spawn of native cutthroats.

The cutthroat is picky about where it spawns; the fish seeks cold mountain streams with just the right amount of water from spring snowmelt.

Last year I was wading in one of those streams with biologist Brad Shepard, from the Wildlife Conservation Society. He was putting electronic tags on the fish to follow them through life. Shepard is worried about their future, because they're under attack. "What we're doing," he told me, is "looking at the invasion front of nonnative fish as they are moving up, as the climate's warming."

The nonnative fish are rainbow trout. They were introduced in Western lakes and streams decades ago to give anglers more fish to catch. The rainbows mostly stayed well down in the valleys, because mountain water at higher elevations — where cutthroat spawn — was too cold, and the amount of water in the streams wasn't right for rainbows.

But over the past few decades, stream flow at higher elevations is down and water temperature is up. These new conditions favor rainbow trout. So rainbows are moving upstream. When they meet the cutthroat, they mate and create a hybrid — essentially, a different type of trout.

"And it's irreversible," says Clint Muhlfeld, a biologist with the U.W. Geological Survey. "Hybrids produce hybrids." In other words, once hybridized, the fish population can't return to what it used to be genetically.

There are still plenty of cutthroats in Montana, Muhlfield says, but now more of them are in danger of becoming hybrids.

Now, you might think, so what? Surely a trout's a trout by any other name. But trout fishing brings in tens of millions of dollars (just in Montana), and lots of anglers want to catch native cutthroats — they're as picky about their trout as the trout are about where they spawn.

More important, biologists like Muhlfeld who have been tracking the reproductive success of these hybrids worry that might die out too, leaving fewer fish overall. Recent genetic evidence that he and his colleagues published this week in the journal Nature suggests the hybrid trout aren't doing well at all.

"The hybrid offspring have greatly reduced fitness," Muhlfeld says — fitness being "their ability to produce offspring and have those offspring survive."

The threat from introduced rainbow trout was held at bay for decades, until the climate changed, he says. "So essentially, hybridization was a time bomb waiting to go off under the right environmental conditions."

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MELISSA BLOCK, HOST:

Many parts of the US have been getting warmer over the past several decades and have also been experiencing persistent drought. Wildlife often can't adjust. NPR's Christopher Joyce reports on new evidence that a changing climate maybe threatening one of the American West most highly prized fish - the cutthroat trout.

(SOUNDBITE OF RUNNING STREAM)

CHRISTOPHER JOYCE, BYLINE: If you wade into one of the tiny streams of the Shields Basin in Montana, you might see a two-inch fish wriggling out from under a rock. These cold mountain streams are home to young cutthroat trout. Last year I was waiting in one of the streams with biologist Brad Shepard from the Wildlife Conservation Society. He was looking for invaders.

BRAD SHEPARD: What we are doing is we're looking at the invasion front of non-native fish as they are moving up, as the climate is warming.

JOYCE: The non-native fish are rainbow trout. They were introduced in the West decades ago to give anglers more fish to catch. The rainbows mostly stayed well down in the valleys - the mountain water was too cold. But over the past few decades, stream flow at higher elevations is down and water temperature is up. These conditions favor rainbow trout, so rainbows are moving upstream. When they meet the cutthroat, they mate and create a hybrid - essentially a new kind of trout.

SHEPARD: And it is irreversible. Hybrids produce hybrids.

JOYCE: Clint Muhlfeld is a biologist with the U.S. Geological Survey. He says there are still plenty of cutthroats, but they are all in danger of becoming hybrids. Now you might think, so what? A trout is a trout by any other name. But trout fishing brings in tens of millions of dollars just in Montana, and lots of anglers want to catch native cutthroat trout. More important, biologist like Muhlfeld worry that these hybrids might die out too, leaving fewer fish overall.

CLINT MUHLFELD: Well, we started studying this, we were first finding all these hybrid trout across all these water bodies and wondering what is hybridization do to their overall performance?

JOYCE: Now Muhlfeld and his colleagues have done the genetic research to answer that question. And in fact these hybrid trout don't do well at all.

MUHLFELD: The hybrid offspring have greatly reduced fitness. Their ability to produce offspring and have those offspring survive.

JOYCE: Muhlfeld says a change in climate triggered this phenomenon.

MUHLFELD: So essentially, hybridization was a time-bomb, waiting to go off under the right environmental conditions.